Heterogeneous polymerization in carbon dioxide

ABSTRACT

The heterogeneous polymerization of water-insoluble polymer in CO2 is disclosed. The method comprises providing a heterogeneous reaction mixture comprising CO2, a monomer, and a surfactant, then polymerizing the monomer to form a water-insoluble polymer.

This application is a continuation application of prior application Ser.No. 08/443,478, filed on 18 May 1995, now U.S. Pat. No. 5,589,105, whichis a divisional of U.S. patent application Ser. No. 08/378,550, filed 25Jan. 1995, now U.S. Pat. No. 5,506,317, which is a divisional of U.S.patent application Ser. No. 08/299,516, filed 1 Sep. 1994, now U.S. Pat.No. 5,451,663, which is a divisional of U.S. patent application Ser. No.08/198,224, filed 17 Feb. 1994, now U.S. Pat. No. 5,382,623, which is adivisional of U.S. patent application Ser. No. 08/099,905, filed 30 Jul.1993, now U.S. Pat. No. 5,312,882.

FIELD OF THE INVENTION

This invention relates generally to the polymerization of hydrophobicmonomers, and more specifically relates to the heterogeneouspolymerization of hydrophobic monomers in a carbon dioxide continuousphase.

BACKGROUND OF THE INVENTION

Emulsion polymerization is a heterogeneous process often used byindustry to polymerize a wide variety of monomers using free radicalmechanisms. It involves the polymerization of monomers in the form ofemulsions or latexes. Polymers commonly formed by emulsion includeacrylics, styrenics, polyvinylchloride (PVC), styrene-butadiene rubber,ethylene-propylene-diene terpolymer-based (EPDM), polystyrene,acrylonitrile-butadiene-styrene copolymer (ABS), neoprene rubber,ethyl-vinyl acetate, styrene-maleic anhydride, tetrafluroethylene, andvinyl fluoride.

Generally, low molar mass ionic surfactants have enjoyed the most use inwater-based emulsion polymerizations because they work so efficiently tostabilize the ionic double layer of the emulsion or colloid particleswhich prevents particle coagulation. In addition, polymeric surfactantshave also been utilized to stabilize emulsion polymerizations. SeePiirma, Polymeric Surfactants in 42 Surfactant Science Series (MarcelDekker, New York 1992). This class of surfactants stabilizes colloidalparticles by steric, rather than ionic, means. Steric stabilization ofemulsions can be advantageous in that (a) steric systems are much lesssensitive to fluctuations and increases in electrolyte concentrations,(b) they work well at high and low solids contents, and (c) theystabilize aqueous and nonaqueous dispersions equally well. See Napper,Polymeric Stabilization of Colloidal Dispersions (Academic Press, NewYork 1983). Many nonionic polymeric surfactants are available; the mostcommon of these are basically block copolymers of poly(ethylene oxide)(PEO) and poly(propylene oxide) (PPO).

After polymerization, the polymer must be coagulated and isolated fromthe aqueous phase for further processing (except for that which isdestined for use in water-borne coatings). The large volume of waterremaining comprising the continuous phase must be properly handled, asit becomes contaminated with organic compounds-residual monomers,stabilizers, and other materials that are difficult to remove. As aresult, it would be desirable to provide a different medium for thecontinuous phase that can be easily decontaminated.

In view of the foregoing, it is a first object of the present inventionto provide a heterogeneous polymerization method in which a fluid otherthan water comprises the continuous phase medium.

It is also an object of the present invention to provide surfactantsuseful for the foregoing methods.

It is a further object of the present invention to provide initiatorssuitable for use with the foregoing methods.

SUMMARY OF THE INVENTION

These and other objects are satisfied by the present invention, whichincludes as a first aspect a method of carrying out the heterogenouspolymerization of monomers that form a water-insoluble polymer. Themethod comprises providing a heterogeneous reaction mixture comprisingcarbon dioxide, a monomer, and a surfactant and polymerizing the monomerin the reaction mixture. The method is suitable for both suspension andemulsion polymerizations.

The present invention includes as a second aspect a heterogeneousreaction mixture useful for carrying out the heterogenous polymerizationof a monomer that forms a water-insoluble polymer. The reaction mixturecomprises a carbon dioxide, the monomer, and a surfactant.

A third aspect of the present invention is a surfactant useful forcarrying out the heterogenous polymerization of a hydrophobic monomer.The surfactant comprises a first hydrophobic group covalently joined toa second carbon-dioxide soluble group, wherein the carbon-dioxidesoluble group comprises a fluorinated siliconated component.

A fourth aspect of the present invention is a polymerization initiatorsuitable for use in a heterogeneous polymerization carried out with acarbon dioxide continuous phase. The initiator comprises a first carbondioxide-soluble group comprising a fluoropolymer covalently joined to asecond free-radical forming group.

The use of CO₂ as the continuous phase medium rather than water isadvantageous for a number of reasons. First, because water has beenreplaced as the continuous phase medium, there is no longer a concernabout contaminating (and thus having to purify) the continuous phasewater. Second, the polymer can be easily isolated from the continuousphase, as the CO₂ can simply be vented from the reaction vessel. Third,the density and hence the viscosity of CO₂ can be tuned over a largerange of conditions due to its compressibility, particularly in thesupercritical phase, and thus particle size and morphology of thepolymer can be controlled.

Carbon dioxide (CO₂) has been employed as a polymerization medium. Forexample, European Patent Application No. 88112198.2, filed 29 Jul. 1988,discloses the polymerization of acrylic acid monomer, a comonomer. CO₂,and an initiator. The CO₂ can be used in a supercritical fluid phase.The resulting. copolymer is useful as a thickening agent. In addition,Hartmann et al., U.S. Pat. No. 4,748,220, disclose a polymerization ofmonoethylenically unsaturated carboxylic acids, their amides and esters,and aminoalcohols in supercritical CO₂. However, neither of thesereferences disclose a heterogenous polymerization reaction in which CO₂is the continuous phase medium and in which a surfactant is included toinduce micelle formation, nor is a surfactant suitable for use in such apolymerization disclosed. Recently, Consani et al., J. Supercrit. Fl.3:51 (1990), reported a screening of the solubility of differentsurfactants in CO₂. Also, it has been shown that a large variety offluoropolymers and fluorinated copolymers are very soluble in CO₂. SeeDeSimone et al., Science 257:945 (1992). None of this suggests the useof CO₂ as a continuous phase in a heterogenous polymerization.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plot showing the ¹ H NMR spectra for an FOA-styrene-FOAtriblock copolymer.

FIG. 2 is a plot showing the absorption spectrum for a FOMA-Ethylhexylacrylate-FOMA copolymer in CO₂.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method of carrying out theheterogeneous polymerization of a hydrophobic monomer. The steps of themethod comprise providing a reaction mixture comprising a carbon dioxide(CO₂) continuous phase, and a hydrophobic dispersed phase comprising asurfactant and a hydrophobic monomer dissolved therein, thenpolymerizing the monomer in the reaction mixture.

As used herein, a "heterogeneous reaction" is one in which thepolymerization is carried out through the use of media that create atleast two separate phases. One phase is termed the "continuous phase",which comprises a fluid, and the other is termed the "dispersed phase",comprising the monomer or monomers to be polymerized. The monomer isstabilized in the dispersed phase by a surfactant (also known as anemulsifier, or a stabilizer) that reduces the surface tension betweenthe phases. The term "heterogeneous reaction" is intended to encompassboth suspension polymerizations, in which any polymerization initiatoris preferentially solubilized in the dispersed phase, and emulsionreactions, in which any polymerization initiator is preferentiallysolubilized in the continuous phase. As used herein, a compound is"preferentially solubilized" in one phase over another when it is moresoluble in that phase.

The present invention is preferably carried out by emulsionpolymerization. The generally accepted view of emulsion polymerizationis described in Harkins, J. Amer. Chem. Soc. 69:1428 (1947), and furtherdeveloped in Smith and Ewart, J. Chem. Phys. 16:592 (1948). A classicaloil-in-water (O/W) emulsion polymerization generally includes asreagents water, a water-insoluble monomer, a water-soluble initiator,and an emulsifier. As the monomer is insoluble in the continuous phase(water), it is dispersed as droplets, which are stabilized by thesurface active emulsifier, and is also solubilized in micelles formed bythe emulsifier. The initiator is soluble in the continuous water phase.Upon its decomposition to form radicals, the initiator initiates thepolymerization of the trace amount of monomer dissolved in thewater-rich phase. As the molecular weight of the macromoleculeincreases, it eventually becomes insoluble in water, at which time itprecipitates to form a primary particle. These primary particles cancoalesce to form larger particles which become stabilized by the surfaceactive emulsifier present in the system.

The overall rate of polymer propagation, R_(p), typically follows theso-called Smith-Ewart kinetics described by the following equation:

    R.sub.p =k.sub.p N<n> M!                                   (I)

where k_(p) is the rate constant for propagation, N is the number ofparticles that form, <n> is the average number of radicals per particle,and M! is the monomer concentration in the particles. The number averagedegree of polymerization, <Xn>, can also be quantified as:

    <Xn>=k.sub.p N M!R.sub.i.sup.-1                            (II)

where R_(i) is the rate of initiation. It is clear that both the rate ofpropagation and the overall molar mass of the resulting polymer are bothdependant on the number of polymer particles, N. According toSmith-Ewart theory, the number of particles is given by:

    N∝ S!.sup.3/5                                       (III)

where S! is the total surfactant or emulsifier concentration. Thissimultaneous dependence of the rate of polymerization and the molar massof the polymer on the surfactant concentration illustrates the utilityof emulsion polymerization processes, as a reaction can simultaneouslyprovide high rates of polymer production and high molar masses; thisresult differs markedly from solution polymerizations that transpire ina single phase.

The CO₂ can be employed in a liquid, vapor, or supercritical phase. Ifliquid CO₂ is used, the temperature of the reaction should be below 31°C. Preferably, the CO₂ in the continuous phase is in a "supercritical"phase. As used herein, "supercritical" means that a fluid medium is at atemperature that is sufficiently high that it cannot be liquified bypressure. The thermodynamic properties of CO₂ are reported in Hyatt, J.Org. Chem. 49:5097-5101 (1984); therein, it is stated that the criticaltemperature of CO₂ is about 31° C.; thus the method of the presentinvention should be carried out at a temperature above 31° C. Thereaction temperature should be chosen to provide sufficient heat energyto initiate and propagate the polymerization. Preferably, the reactiontemperature will be between 50° C. and 200° C., and more preferably willbe between 50° C. and 100° C.

The advantage of conducting the polymerization with supercritical CO₂inures from the tendency of the solvent strength of a solvent in asupercritical phase to be easily manipulated by varying the pressure ofthe fluid. As a result of this phenomenon, the use of supercritical CO₂permits one carrying out the polymerization to significantly influencethe particle size, distribution, and other aspects of the final productwithout varying either the solvent temperature or composition (i.e.,including a co-solvent).

The method of the present invention includes a dispersed phasecomprising a monomer stabilized by a surfactant. The surfactant providedto the reaction mixture should be one that is surface active in CO₂ andthus partitions itself at the CO₂ -monomer/polymer interface. Such asurfactant should cause the formation ok micelles in the CO₂ and thuscreate a dispersed phase that permits the polymerization to follow theSmith-Ewart kinetics described above. The surfactant is generallypresent in the reaction mixture in a concentration of between 0.01 and30 percent by weight. The surfactants can be nonreactive in thepolymerization or can react with an thereby be included with the formingpolymer. See, e.g., U.S. Pat. Nos. 4,592,933 and 4,429,666 for exemplaryreactive surfactants.

The surfactant should contain a segment that is soluble in CO₂ ("CO₂-philic"). Exemplary CO₂ -philic segments include a fluorine-containingsegment or a siloxane-containing segment. As used herein, a"fluoropolymer" has its conventional meaning in the art. See generallyFluoropolymers (L. Wall, Ed. 1972)(Wiley-Interscience Division of JohnWiley & Sons); see also Fluorine-Containing Polymers, 7 Encyclopedia ofPolymer Science and Engineering 256 (H. Mark et al. Eds., 2d Ed. 1985).Exemplary fluropolymers are those formed from: fluoroacrylate monomerssuch as 2-(N-ethylperfluorooctanesulfonamido) ethyl acrylate("EtFOSEA"), 2-(N-ethylperflooctanesulfonamido) ethyl methacrylate("EtFOSEMA"), 2-(N-methylperfluorooctanesulfonamido) ethyl acrylate("MeFOSEA"), 2-(N-methylperflooctanesulfonamido) ethyl methacrylate("MeFOSEMA"), 1,1-Dihydroperfluorooctyl acrylate ("FOA"), and1,1-Dihydroperfluorooctyl acrylate ("FOMA"); fluoroolefin monomers suchas tetrafluoroethylene, fluorostyrene monomers such as α-fluorostyrene,β-fluorostyrene, α,β-difluorostyrenes, β,β-difluorostyrenes,α,β,β-trifluorostyrenes, α-trifluoromethylstyrenes,2,4,6-Tris(trifluoromethyl)styrene, 2,3,4,5,6-pentafluorostyrene,2,3,4,5,6-pentafluoro-α-methylstyrene, and2,3,4,5,6-pentafluoro-β-methylstyrene; fluoroalkylene oxide monomerssuch as perfluoropropylene oxide and perfluorocyclohexene oxide;fluorinated vinyl alkyl ether monomers; and the copolymers thereof withsuitable comonomers, wherein the comonomers are fluorinated orunfluorinated. Exemplary siloxane-containing compounds include alkyl,fluoroalkyl, and chloroalkyl siloxanes.

More preferably, the surfactant comprises a hydrophobic group, such as apolystyrene group, that is "CO₂ -phobic," along with a CO₂ -solublegroup, such as a fluoropolymer. Such copolymers can take many forms; twoexemplary forms are graft copolymers having a CO₂ -soluble backbone andhydrophobic branches attached thereto and triblock copolymers having acentral hydrophobic portion attached at opposite ends to one of a pairof CO₂ -soluble portions are preferred. Triblock copolymers oftenexhibit markedly different properties than their individualsconstituents, as the individual segments of the copolymers tend to phaseseparate; the chemical bond between the segments prevents macroscopicphase separation, so microdomains tend to form. When a triblockcopolymer as described above is dissolved in CO₂, the CO₂ -soluble endportions extend into the CO₂ continuous phase, but the hydrophobicportions can aggregate to form the core of a micelle. It is particularlypreferred that the fluoropolymer segment be a perfluoropolymer such asdihydroperfluorooctyl acrylate. A preferred triblock copolymersurfactant comprises poly(1,1-dihydroperfluorooctyl acrylate) end blocksand a polystyrene center block.

Other suitable surfactants that are surface active in carbon dioxide tostabilize the dispersed phase include ##STR1## where x=1-30 and y=1 to30. The x and y values are chosen to adjust the balance between "CO₂-philic" and "CO₂ -phobic" to tailor. The surface activity of thereagents. Exemplary silicone-containing surfactants include ##STR2##wherein x and y are varied to adjust to "CO₂ -philic" and "CO₂ -phobic"balance.

Suitable monomers for use with this invention are those that formwater-insoluble polymers. Exemplary monomers forming such polymersinclude vinyl monomers such as vinyl chloride and vinyl acetate,ethylene, acrylonitrile, dienes such as isoprene and butadiene,styrenics such as styrene and t-butyl styrene, chloroprene, acrylicmonomers such as alkylmethyl acrylates, fluromonomers as given above,and maleic anhydride. Particularly suitable hydrophobic monomers can beselected from the group consisting of styrene monomers, acrylicmonomers, vinyl chloride monomers, olefinic monomers, fluoroolefinicmonomers, fluoroacrylate monomers, fluorostyrenic monomers, and maleicanhydride monomers. The method is suitable for polymerizations of asingle monomer or copolymerizations of more than one hydrophobicmonomer, and can also be end to copolymers hydophobic and hydrophilicmonomers. In addition, monomers that provide cross-linking andbranching, such as divinyl benzene and di-triacrylates, can also beincluded. The monomer can be included in the reaction mixtureproportions ranging from 1 to 70 percent by weight.

The heterogeneous reaction can optionally include a free radicalinitiator that accelerates the polymerization. The initiator is chosenbased on its solubility characteristics: it should preferentiallysolubilize in the dispersed phase for a suspension reaction, or in theCO₂ continuous phase for an emulsion reaction. The initiator is includedin the solution in a concentration ranging from 0.001 to 20 percent byweight.

Those skilled in this art will be familiar with a number of initiatorsthat can solubilize in hydrophobic media. Organic free radicalinitiators are preferred and include, but are not limited to, thefollowing: acetylcyclohexanesulfonyl peroxide; diacetylperoxydicarbonate; dicyclohexyl peroxydicarbonate; di-2-ethylhexylperoxydicarbonate; tert-butyl perneodecanoate; 2,2'-azobis(methoxy-2,4-dimethylvaleronitrile; tert-butyl perpivalate; dioctanoylperoxide; dilauroyl peroxide; 2,2'-azobis (2,4-dimethylvaleronitrile);tert-butylazo-2-cyanobutane; dibenzoyl peroxide; tert-butylper-2-ethylhexanoate; tert-butyl permaleate; 2,2-azobis(isobutyronitrile); bis(tert-butylperoxy) cyclohexane; tert-butylperoxyisopropylcarbonate; tert-butyl peracetate; 2,2-bis(tert-butylperoxy) butane; dicumyl peroxide; ditert-amyl peroxide;di-tert-butyl peroxide; p-methane hydroperoxide; pinane hydroperoxide;cumene hydroperoxide; and tert-butyl hydroperoxide. Preferably, theinitiator is azobisisobutyronitrile ("AIBN").

Initiators that preferentially solubilize in CO₂ include a CO₂ -philicsegment (typically a fluorinated or siloxane segment) and a freeradical-producing segment; thus the attachment of a fluorinated orsiloxane segment to the free radical initiators listed above producesinitiators that can be used with the present invention. A particularlypreferred CO₂ - soluble initiator is that of Formula XIII: ##STR3##

The polymerizing step of the present invention can be carried out bypolymerization methods using apparatus and conditions known to thoseskilled in this art. Typically, the reaction begins by heating thereaction vessel to a temperature above 31° C. (generally between 50° C.and 200° C.). The initiator, monomer or monomers, surfactant, and CO₂,are added to the vessel. Typically the mixture is allowed to polymerizefor between about 2 and 24 hours, and preferably is stirred as thereaction proceeds. At the conclusion of the reaction, the polymer can becollected by methods such as venting of the CO₂ or by fractionation.Preferably, the surfactant is fractionated from the CO₂ and polymer byreducing temperature and pressure and thus is able to be reused. Afterseparation, the polymer can be collected by conventional means. Inaddition, the polymers of the present invention may be retained in theCO₂, dissolved in a separate solvent, and sprayed onto a surface. Afterthe CO₂ and solvent evaporate, the polymer forms a coating on thesurface.

The polymer formed by the present invention can also be used to formmolded articles, such as valves and bottles, films, fibers, resins, andmatrices for composite materials.

The present invention is explained in greater detail in the followingexamples. As used herein, "M" means molar concentration, "NMR" meansnuclear magnetic resonance, "MHz" means megahertz, "GPC" mean gas phasechromatography, "Å" means angstroms, "UV" means ultraviolet, "g" meansgrams, "mol" means moles, "mL" means milliliters, "°C." means degreesCelsius, "S" means seconds, "h" means hours, "psig" means pounds persquare inch (gauge), "M_(n) " means number merger molecular weight,"MWD" means molecular weight distribution, "ƒ" means functionality,"ppm" means parts per million, "T_(g) " means glass transitiontemperature, "nm" means nanometers, "mg" means milligrams, "rpm" meansrevolution per minute, and "psi" means pounds per square inch. TheseExamples are illustrative and are not to be taken as limiting of theinvention.

EXAMPLE 1 Reagents and Materials

The preparation of the reagents and materials used in the subsequentexamples is set forth hereinbelow. 4,4'-Azobis-4-cyanopentanoic acid,potassium t-butoxide (1M in THF), and a,a,a-trifluorotoluene (99+% pure)(Aldrich), sodium bicarbonate and sodium sulfate (Fisher Scientific),acetone (EM Science, GR grade), methanol and hexanes (Mallinckrodt,Inc., HPLC grade), 1,1,2-Trifluorotrichloroethane (3M) (Freon-113),sec-Butyllithium (1.4M in cyclohexane--provided courtesy of LithiumCorporation), chlorosilane perfluorohexylethyl-dimethyl silylchloride(Petranch), and perfluorinated alcohol (DuPont) were used as received.Pyridine (EM Science) was purified by vacuum distillation.Tetrahydrofuran (Fisher Certified Grade) (THF) was refluxed over sodiumand distilled under an argon atmosphere. Acryloyl chloride (Aldrich) waspurified by vacuum distillation. Azobisisobutyro-nitrile (Kodak) (AIBN)was recrystallized from methanol. 1,1-Dihydroperfluorooctyl acrylate(3M) (FOA) was passed through columns of decolorizing carbon andalumina. Cyclohexane (Phillips Petroleum Company) was stirred overconcentrated sulfuric acid for approximately two weeks, decanted anddistilled from a sodium dispersion under argon. Styrene (Fisher) wasvacuum distilled from dibutyl magnesium following three freeze-thawcycles. p-Vinylbenzyl chloride (Kodak) was passed through an aluminacolumn under argon atmosphere. p-Vinylbenzyl iodide was synthesized fromp-vinylbenzyl chloride using the Finkelstein reaction. Carbon dioxide(Matheson, 99.5%) was purified by passing through columns of molecularsieves and reduced copper oxide catalyst (BASF R3-11). Tetraethylthiuramdisulfide (TD, Aldrich) was recrystallized twice from methanol and thepurity was checked by ¹ H NMR. Toluene (Fisher, Certified Grade) wasdistilled under argon over sodium metal. 1,1-Dihydroperfluorooctylmethacrylate (3M) (FOMA) and 2-ethylhexyl acrylate (Aldrich) and styrene(Aldrich) were passed through a column of alumina prior to use. Allglassware was rigorously cleaned and flame-dried prior to use.

EXAMPLE 2 Characterization of Polymers

¹ H NMR spectra were recorded on a Bruker AMX300 NMR spectrometer or ona Bruker AC-200 at 200 MHz in CDCl₃ or Freon-113/CDCl₃. UV spectra wereobtained on a Perkin Elmer Lambda 6 UV/vis spectrometer interfaced to anIBM PS/2 model 50 computer. Differential scanning calorimetry analyseswere performed on a Perkin Elmer DSC-7. The molecular data of thetelechelic polystyrenes were obtained by running GPC on a Waters 150-CVgel permeation chromatograph with Ultrastyragel columns of 100, 500,10³, 10⁴, and 10⁵ Å porosities using THF as eluent. Polystyrenestandards (Showa Denko) were used to determine the molecular mass andmolecular weight distribution. Macromolecular end groups were analyzedby ¹ H NMR, UV analyses, and by GPC analyses on another Waters GPCinstrument with a Waters 996 photodiode array detector which measuresthe UV spectrum of each elution of the polymer.

EXAMPLE 3 Fluoro-azo Initiator Synthesis

4,4'-Azobis-4-cyanopentanoyl chloride was prepared from the acid analogby the method described in Smith, Makromol. Chem. 103:301 (1967). 0.022mol (7.124 g) of 4,4'-azobis-4-cyanopentanoyl chloride in 160 mL of dryTHF was added drop-wise to a solution of 0.039 mol (20.302 g) ofperfluorinated alcohol and a catalytic amount of pyridine in 60 ml ofFreon 113 at room temperature and stirred for 3 hours under inertatmosphere. After the filtering of pyridine hydrochloride, the solutionwas concentrated and washed with a NaHCO₃ solution and water to removeunreacted 4,4'-azobis-4-cyanopentanoic acid and pyridine. The organiclayer was dried over Na₂ SO₄ and solvent was evaporated under reducedpressure to produce a 70 percent yield of a compound of Formula XIII:##STR4##

EXAMPLE 4 Fluoro-Azo Initiator Solubility

The initiator used in an emulsion polymerization should bepreferentially soluble in CO₂ and partition itself into the continuousCO₂ phase over the hydrophobic dispersed phase in order to benefit fromSmith-Ewart kinetics. The fluorinated azo-initiator synthesized inExample 3 meets this solubility criterion. This molecule is very solublein CO₂ and decomposes in an analogous fashion to AIBN. The initiatordecomposes with a first order rate constant of k_(d) =15.64×10⁻⁶ s⁻¹which indicates that it has a 12.3 h half life at 70° C. The highlyfluorinated nature of this molecule imparts the desirablesolubility/insolubility profile; it is insoluble in most organicsolvents (benzene, toluene, cyclohexane, acetonitrile,carbontetrachloride, dimethylformamide, dimethylacetamide); insoluble inmany hydrophobic monomers (styrene, t-butyl styrene, acrylic acid);insoluble in water; and soluble in Freon-113 and CO₂.

EXAMPLE 5 Polystyrene Macromonomer Synthesis

A. Anionic polymerization: The anionic polymerization of styrene wasconducted in a one-neck 500 mL round-bottomed flask equipped with amagnetic stir bar and rubber septum under a 5-8 psig argon atmosphere.The polymerization was initiated with sec-butyllithium and stirredovernight at room temperature. The polymerization was functionallyterminated by the addition of a two-fold excess of ethylene oxidefollowed by the addition of vinylbenzyl iodide. The polymer wasprecipitated in a ten-fold excess of methanol, dissolved in THF toremove unreacted vinylbenzyl iodide and reprecipitated in methanol. Thepolymer was then dried in vacuo overnight and stored at -8° C.

B. Free Radical Polymerization of Telechelic Polystyrene: Telechelicdithiocarbamate functionalized polystyrenes of different molecularweight were prepared by the so-called "iniferter" technique using TD asthe iniferter. See Ostu et al., Makromol. Chem Rapid Commun. 3:127(1982). Previous studies have shown that TD not only serves as a freeradical initiator, but also has high reactivity for chain transfer toinitiator and primary radical termination. Id. These features ensurethat the polymer will be end-capped with two initiator fragments.

After polymerization, the polymer was recovered by precipitation of thepolymerization solution into a large excess of methanol and drying. Theresulting polymer was purified twice by dissolution in THF andreprecipitation into methanol.

The telechelic polystyrene produced by this method was thencharacterized (see Table 1). The molecular weight of the polymer wasdetermined by GPC. The presence of the residual initiator and thefunctionality of the end groups were determined by ¹ H NMR and by UVanalyses as described previously. See Turner et al., Macromolecles23:1856-1859 (1990).

                  TABLE 1                                                         ______________________________________                                        Synthesis of Telechelic Polystyrenes                                          Sample ID M.sub.n (GPC)  MWD     f(%)                                         ______________________________________                                        1         3.3K           1.5     1.9                                          2         5.6K           1.8     1.8                                          3         8.8K           2.1     2.0                                          ______________________________________                                    

The telechelic polystyrenes were also analyzed by a Waters GPC with aphotodiode array detector which can give an elution time-peakintensity-UV absorption spectra 3-dimensional plot. The functionalitycan also be calculated from the 3-D GPC plot.

EXAMPLE 6 Synthesis of Graft Polymer Surfactant by Copolymerization ofPolystyrene Macromonomer and FOA

A calculated amount of PS macromonomer as synthesized in Example 5.A,FOA and AIBN were added to a round bottom flask and deoxygenated. Thesynthesis is depicted in Scheme 1 below. ##STR5## A mixture of Freon-113and THF were added under argon and the flask was placed in a water bathat 60° C. for approximately 48 h. The PFOA-g-PS copolymer wasprecipitated in methanol, extracted several times with cyclohexane, anddried to constant weight at ambient temperature in vacuo.

The copolymer produced by this method is a graft copolymer having a "CO₂-philic" PFOA backbone with hydrophobic PS branches. After extraction toremove any unincorporated macromonomer, the poly(FOA-g-PS) copolymer issoluble in CO₂ at 3500 psi, 60° C. (10 wt %).

EXAMPLE 7 Synthesis of FOA-Styrene-FOA Triblock Copolymer Surfactants

1 g of telechelic polystyrene (M_(n) =5590, MWD=1.8) as synthesized inExample 5.B was used as the photoinitiator of FOA monomer (5 g) ina,a,a-trifluorotoluene solution (20 mL). Three different samples wereprepared by Scheme 2 below. ##STR6## In Sample 1, m=25.5, n=51; inSample 2, m=79, n=51; in Sample 3, m=50.9 , n=28.7. Upon UV irradiationof the functionalized polystyrene (Hanovia, 140W), the chain endsdissociate to generate polymeric radicals which in turn initiate thepolymerization of FOA. The dithiocarbamate radical is less reactive andis not effective in initiating the polymerization of acrylate monomers.After 48 h UV irradiation, the polymerization solution was precipitatedinto methanol to give 4.88 g of polymer (81.3% yield).

EXAMPLE 8 Characterization of FOA-Styrene-FOA Triblock Copolymer

The block copolymers prepared in Example 7 were purified by Soxheletextraction with cyclohexane to remove any unreacted prepolymer. Thecopolymer of sample 1, with the short FOA block, is soluble in THF,CHCl₃ and hot cyclohexane. About 5 wt % of the polymer product issoluble in cyclohexane at room temperature which was found to be by ¹ HNMR to be mainly unreacted polystyrene. Samples 2 and 3, with longer FOAblocks, were Soxhlet extracted with hot cyclohexane. As determined by ¹H NMR, the hot cyclohexane extract removed not only the unreactedpolystyrene, but also some block copolymer that was low in FOAcomposition. The synthesis data are summarized in Table 2:

                  TABLE 2                                                         ______________________________________                                        Synthesis of ABA Triblock Copolymers*                                         TD-PSt        FOA    block copolymer                                          run M.sub.n f     wt (g)                                                                              (g)  wt (g) yield(%)                                                                             M.sub.n (× 10.sup.4)**       ______________________________________                                        1   5.6K    1.8   1.0   5.0  4.88   81     2.87                               2   5.6K    1.8   0.5   5.0  4.89   89     7.73                               3   3.3K    1.9   0.2   5.0  4.43   85     4.95                               ______________________________________                                         *Polymerization run overnight in tribluorotoluene/freon113 (5/1) mixed        solvent with a 140W UV lamp as irradiation source.                            **Determined from .sup.1 H NMR and the M.sub.n of the prepolymer.        

The purified triblock polymers were characterized with ¹ H NMR and DSC.¹ H NMR spectra (FIG. 1) show the resonances of both styrene and FOArepeating units. The peak at 4.61 ppm is due to the methylene of theester group of FOA and the aromatic resonances (6.3-7.3 ppm) are due tothe phenyl ring of styrene. From the ratio of the area of the two peaksthe chemical composition and the molecular mass was calculated (Table2).

The DSC trace of Sample 1 shows two glass transition temperatures whichindicates microphase separation in the bulk. T_(g) ¹ =-10° C.corresponds to the glass transition of FOA microdomains and T_(g) ² =97°C. corresponds to the glass transition of the styrene micro-domains.

As shown in Table 3, the solubility of the block copolymers are quitedifferent from the homopolymers. The FOA homopolymer is soluble inFreon-113 and CO₂ but insoluble in common organic solvents. However, thesample 1 block copolymer is soluble in THF, CHCl₃, etc. As thefluorinated block becomes longer (sample 2) or the center block becomeshorter (sample 3), the copolymer is insoluble in THF or CHCl₃, but issoluble in Freon-113 and CO₂.

                  TABLE 3                                                         ______________________________________                                        Solubility of Triblock Copolymer in Different Solvents                                       Solubility                                                     Sample   n      m        THF  CHCl.sub.3                                                                            Freon                                                                              CO.sub.2                           ______________________________________                                        1        51     25.5     +    +       +    -                                  2        51     79       -    -       +    +                                  3        28.7   50.9     -    -       +    +                                  ______________________________________                                         "+" = soluble; "-" insoluble.                                            

EXAMPLE 9 Synthesis of FOMA-2-Ethylhexyl Acrylate-FOMA TriblockCopolymer Surfactant

A 450 mL Autoclave Engineers o-ring closure, stirred autoclave with amodular furnace and process controller was used as the high pressurepolymerization reactor. Pressure was measured with Sensotec Model TJEpressure transducer. Three rupture disks with a burst pressure of 690bar are present in the system and all of the components were connectedto argon to maintain an inert atmosphere at all times.

The reactor was heated to 60° C. and purged with argon for 1-2 hours.1.35 g (1 mol % to monomer) of AIBN dissolved in a minimal amount of THFwere injected into the reactor. Following the rapid evaporation of THF,99.5 g of FOMA (50 mol %) and 35.5 g of 2-Ethylhexyl acrylate (50 mol %)were introduced under argon atmosphere. The reactor was then rapidlypressurized to 345 bar with CO₂. The reaction was allowed to stir for 24hours. The reactor was then cooled to room temperature and the polymerwas collected into an erlenmeyer flask, washed with methanol and driedin vacuo overnight (yield 75%).

EXAMPLE 10 Solvatochromic Studies of Phenol Blue in CO₂ With and WithoutFOMA-2-Ethylhexyl Acrylate-FOMA Triblock Copolymers

Solvatochromatic studies were conducted on the copolymer surfactantprepared in Example 9 to investigate micelle formation in CO₂. A dilutesolution of phenol blue in cyclohexane (1.5×10⁵ M) was prepared inadvance. Ten drops of the solution were syringed into a 2.5 mL highpressure UV cell which was loaded with 0.125 g of the triblock copolymer(5 wt. %). The solvent was evaporated by purging with argon. The cellwas filled with carbon dioxide to 3500 psi at room temperature and UVspectra were recorded after the system became homogeneous. Forcomparison, UV spectra of phenol blue in pure CO₂ and CO₂ /poly(FOA)solution were also measured.

EXAMPLE 11 Results of Solvatochromatic Studies of Phenol Blue in CO₂With and Without the FC-HC-FC Triblock Copolymers

Since phenol blue itself is soluble in CO₂, in the micelle solution, thedye should partition between the bulk CO₂ phase and the core of themicelle. The measured λ_(max) of phenol blue in pure CO₂ at 3500 psi androom temperature was 538 nm, which is consistent with the reportedvalue. No shift of the λ_(max) of phenol blue was observed in 5 wt. %Poly(FOA)/CO₂ solution at the same temperature and pressure. However, in5 wt. % triblock copolymer/CO₂ solution, the λ_(max) of phenol blueshifted from 538 nm to 550 nm, and the peak was broadened (FIG. 2). Thissuggests the formation of micelles of the triblock copolymer in carbondioxide, although the broadening of the peak rather than the appearanceof an entirely new peak is likely due to the different shapes and sizesof the micelles formed, and probably also due to the dynamic feature ofthe micelles.

EXAMPLES 12-20 Heterogeneous Polymerization of Styrene with AIBN andwithout Surfactant

Trials attempting to heterogeneously polymerize styrene in CO₂ werecarried out. Parameters varied between polymerization trials were thepressure of the reactor (97 or 345 bar), the initiator (either AIBN (155mg-0.2 wt % to monomer) or the fluoro-azo initiator prepared in Example3 (2.4 g)), and the presence or absence of the FOMA-styrene-FOMAsurfactant prepared in Example 9.

The polymerizations commenced by first heating a high pressure reactorto 60° C. (with AIBN as the initiator) or 75° C. (with the fluoro-azoinitiator) and purging with argon for 1-2 hours. The initiator wasdissolved in 77.5 g (17 wt %) of degassed styrene. The solution wasrapidly added to the reactor followed by the addition of CO2 to thedesired pressure. The reaction was stirred for 8 hours at a rate of 500rpm. Following cooling of the reactor, the polymer was collected, washedwith methanol and dried in vacuo overnight.

The results of the polymerization trials are included in Table 4.

                  TABLE 4                                                         ______________________________________                                        Sample   Pressure                                                                              Surfactant         Yield                                     ID       (bar)   (w/v %)     Initiator                                                                            (%)                                       ______________________________________                                        12       345     none        AIBN    7                                        13       97      none        AIBN   16                                        14       345     5           AIBN   22                                        15       345     none        fluoro-azo                                                                           16                                        16       97      none        fluoro-azo                                                                            9                                        17       345     5           fluoro-azo                                                                           20                                        18       97      1           fluoro-azo                                                                           35                                        19       97      5           fluoro-azo                                                                           45                                        20       97      10          fluoro-azo                                                                           60                                        ______________________________________                                    

As shown by the data of Table 4, the yield of the polymer increasedsubstantially with the inclusion of the fluoro-azo initiator and asurfactant having a CO₂ -philic portion.

EXAMPLE 21 Synthesis of PS-Si(CH₃)₂ CH₂ CH₂ C₆ F₁₃

Anionic polymerization was conducted in a one-neck 500 ml,round-bottomed flask equipped with a magnetic stir bar and rubber seplaunder a 6-8 psig. argon atmosphere. The flask was charged with 2.25 mLof styrene in about 100 mL of dry cycolohexane. The polymerization wasthen initiated by the addition of 3.63 mL of a 1.38M solution ofsec-butyllithlum in cyclohexane. The resulting orange-red solution wasthen allowed to stir for about two hours after which about 2 mL of drytetrahydrofuran was added to the solution. To functionally terminate thepolymerization, 3.31 mL of perfluorohexylethyidimethylsllylchloride (20%excess) was added via syringe and allowed to stir for ca. 30 minutes.During this time, the solution had become cloudy due to theprecipitation of lithium chloride. The polymer was precipitated in aten-fold excess of methanol and washed several times with methanol andwater. The polymer was then dried under reduced pressure at 40° C. fortwelve hours.

The resulting polymer was soluble in CO₂ and 60° C. and 5000 psi. Theprecursor polystyrene backing the fluorinated group was less soluble inCO₂.

The foregoing examples are illustrative of the present invention, andare not to be construed as limiting thereof. The invention is defined bythe following claims, with equivalents of the claims to be includedtherein.

That which is claimed is:
 1. A mixture comprising carbon dioxide and asurfactant, wherein said surfactant comprises a carbon dioxide solublesegment and a CO₂ -phobic group, wherein said carbon dioxide solublesegment is a fluorine-containing segment, wherein said CO₂ -phobic groupis hydrophobic, and wherein said fluorine-containing segment has aterminal perfluorinated group.
 2. The mixture according to claim 1,wherein said carbon dioxide comprises liquid carbon dioxide.
 3. Themixture according to claim 1, wherein said carbon dioxide comprisessupercritical carbon dioxide.
 4. The mixture according to claim 1,wherein said surfactant is selected from the group consisting of:

    CF.sub.3 --(CH.sub.2 .sub.x --(CH.sub.2).sub.y --CH.sub.3  (IV)

    CF.sub.3 --(CF.sub.2).sub.x --CH═CH--(CH.sub.2).sub.y --CH.sub.3(VI)

and ##STR7## wherein x is 1-30 and y is 1-30.
 5. The mixture accordingto claim 1, wherein said CO₂ -phobic group aggregates to form micellesin said carbon dioxide.
 6. The mixture according to claim 1 furthercomprising a cosolvent.
 7. The mixture according to claim 1, furthercomprising a second surfactant, said second surfactant comprising acarbon dioxide soluble segment.
 8. A mixture comprising carbon dioxideand a surfactant, wherein said surfactant comprises a carbon dioxidesoluble segment and a CO₂ -phobic group covalently linked to said carbondioxide soluble segment, wherein said carbon dioxide soluble segmentfurther comprises a fluoropolymer; wherein said surfactant is acopolymer selected from the group consisting of a graft copolymer and ablock copolymer;wherein said carbon dioxide comprises liquid carbondioxide; and wherein said CO₂ -phobic group is hydrophobic.
 9. Themixture according to claim 8, wherein said CO₂ -phobic group aggregatesto form micelles in said carbon dioxide.
 10. The mixture according toclaim 8, wherein said CO₂ -phobic group comprises polystyrene.
 11. Themixture according to claim 8, wherein said surfactant comprises acopolymer of a fluoropolymer and polystyrene.
 12. The mixture accordingto claim 8, further comprising a second surfactant, said secondsurfactant comprising a carbon dioxide soluble segment.
 13. A mixturecomprising carbon dioxide and a surfactant, wherein said surfactantcomprises a carbon dioxide soluble segment, wherein said surfactantfurther comprises a CO₂ -phobic group covalently linked to said carbondioxide soluble segment; wherein said carbon dioxide soluble segmentfurther comprises a siloxane segment;wherein said carbon dioxidecomprises liquid carbon dioxide; and wherein said CO₂ -phobic group ishydrophobic.
 14. The mixture according to claim 13, wherein said CO₂-phobic group aggregates to form micelles in said carbon dioxide. 15.The mixture according to claim 13, wherein said CO₂ -phobic groupcomprises polystyrene.
 16. The mixture according to claim 13, whereinsaid surfactant comprises a copolymer comprising a carbon dioxidesoluble group and a CO₂ -phobic group.
 17. The mixture according toclaim 13, further comprising a second surfactant, said second surfactantcomprising a carbon dioxide soluble segment.
 18. A mixture comprisingcarbon dioxide and a surfactant, wherein said surfactant comprises acarbon dioxide soluble segment and a CO₂ -phobic group covalently linkedto said carbon dioxide soluble segment, wherein said carbon dioxidesoluble segment further comprises a fluoropolymer and wherein saidsurfactant is a copolymer selected from the group consisting of a graftcopolymer and a block copolymer;wherein said carbon dioxide comprisesliquid carbon dioxide; and wherein said CO₂ -phobic group ishydrophobic; said mixture further comprising a cosolvent.
 19. Themixture according to claim 18, wherein said CO₂ -phobic group aggregatesto form micelles in said carbon dioxide.
 20. The mixture according toclaim 18, wherein said CO₂ -phobic group comprises polystyrene.
 21. Themixture according to claim 18, wherein said surfactant comprises acopolymer of a fluoropolymer and polystyrene.
 22. The mixture accordingto claim 18, further comprising a second surfactant, said secondsurfactant comprising a carbon dioxide soluble segment.
 23. A mixturecomprising carbon dioxide and a surfactant, wherein said surfactantcomprises a carbon dioxide soluble segment, wherein said surfactantfurther comprises a CO₂ -phobic group covalently linked to said carbondioxide soluble segment, and wherein said carbon dioxide soluble segmentfurther comprises a siloxane segment;wherein said carbon dioxidecomprises liquid carbon dioxide; and wherein said CO₂ -phobic group ishydrophobic; said mixture further comprising a cosolvent.
 24. Themixture according to claim 23, wherein said CO₂ -phobic group aggregatesto form micelles in said carbon dioxide.
 25. The mixture according toclaim 23, wherein said CO₂ -phobic group comprises polystyrene.
 26. Themixture according to claim 23, wherein said surfactant comprises acopolymer comprising a carbon dioxide soluble group and a CO₂ -phobicgroup.
 27. The mixture according to claim 23, further comprising asecond surfactant, said second surfactant comprising a carbon dioxidesoluble segment.